function my_clock(el){
  var today=new Date();
  var h=today.getHours();
  var m=today.getMinutes();
  var s=today.getSeconds();
  m=m>=10?m:('0'+m);
  s=s>=10?s:('0'+s);
  el.innerHTML = h+":"+m+":"+s;
  setTimeout(function(){my_clock(el)}, 1000);
}

var clock_div = document.getElementById('clock_div');
my_clock(clock_div);